In late years, what is called optical wiring is gaining attention, which can be expected to have effects in power consumption, transmission distance, transmission speed, degree of freedom in design (communication specifications, wiring layouts, electromagnetic noise), wiring space, and the like. Utilization of the optical wiring to super computers, high-performance servers, mobile phones, and the like is considered.
As an optical unit used for the optical wiring, there is used a structure to face-down mount an optical element (VCSEL/PD) or an optical module (optical part) incorporating an optical element and a control IC (Driver/AMP) on a base having an optical waveguide sheet. In this optical unit, it is necessary that a mirror formed in the optical waveguide sheet and the optical axis of a light emitting part or a light receiving part of the optical part are coupled with high accuracy and electrically joined to electrical wirings.
When the optical part is face-down mounted on a base, an underfill material is filled and sealed between the optical part and the base. In this case, a transparent material with high light transmittance is used as the underfill material in order to obtain optical coupling between the optical part and the optical waveguide sheet of the base.    Patent Document 1: Japanese Laid-open Patent Publication No. 2011-96755    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-179578
It is desired for the underfill material used in the optical unit to satisfy both suppression of optical coupling loss due to optical absorption, scattering, mixing in of foreign objects, and the like, and acquisition of joint reliability in the electrically joined region between the optical part and the base. However, in order to obtain excellent optical coupling, high transparency (for example, transmittance ≧about 90%) is needed in the underfill material, and thus there are constraints that the filler cannot be contained in the underfill material, and the like. Thus, there arises a problem that the joint reliability of the electrically joined region cannot be acquired.
To address this problem, there has been proposed a method of filling so that an area including the optical axis of the optical part between the optical part and the base becomes a gap by using an underfill material of high-viscosity resin. In this case, there are a method to fix the side face of an optical part mounted on a base, a method to fix four corners of the optical part, and the like. However, in this case, it is difficult to securely cover the electrically joined region between the optical part and the base with the underfill material, and there occur problems that the electrically joined region is exposed, that entrance of the underfill material into the gap cannot be suppressed, and the like.
There has also been considered filling so that the area including the optical axis of the optical part between the optical part and the base becomes a gap by using an underfill material in a sheet form. However, in this case, it is difficult to adequately deal with a minute optical part, and there occurs a problem that the gap cannot be controlled to be a desired state.